Recently, Levenson masks have become popular as photomasks suitable for ultra-fine fabrication. FIGS. 7A to 7D are plan views showing a conventional Levenson mask and a method for forming a fine pattern using this Levenson mask. Hereinafter, the conventional Levenson mask and the method using the same will be described with reference to these drawings. In the drawings of the present application, circuit patterns, photoresist patterns and shielding regions are shown with oblique lines, shifter regions are shown with dots and non-shifter regions are shown in white.
A conventional Levenson mask 80 shown in FIG. 7B has a pattern in which shielding regions 811-814 are sandwiched by shifter regions 821-823 and non-shifter regions 831-833, respectively. Light transmitting through the shifter regions 821-823 has opposite phase to that of the light transmitting through the non-shifter regions 831-833. Accordingly, these lights interface with each other at the back of the shielding regions 811-814, thereby enhancing fabrication accuracy of the shielding regions 811-814.
FIG. 7A is a plan view showing a circuit pattern (design pattern) 84 of a gate of a transistor. FIG. 7C is a plan view showing a general mask 85 for trimming. FIG. 7D is a plan view showing a photoresist pattern 86 after exposure and development. Hereinafter, a method for forming a fine pattern by using the Levenson mask 80 will be described.
First, the same photoresist film (not shown) is exposed to light using the Levenson mask 80 to form the finest pattern, i.e., the gate pattern, and then exposed using the general mask 85 to form other patterns accompanying the gate. Subsequent development of the exposed photoresist film gives the photoresist pattern 86 where exposed regions of the photoresist film have been removed.
However, the conventional Levenson mask 80 has a drawback of poor fabrication accuracy due to the effect of optical proximity. Optical proximity refers to a phenomenon where the photoresist pattern 86 does not accurately follow the dimensions of the shielding regions 811-814 depending on the patterns of the shifter regions 821-823 and the non-shifter regions 831-833 sandwiching the shielding regions 811-814. For example, a width 87 in the photoresist pattern 86 corresponding to the shielding region 812 is significantly wider than the designed dimensions due to an aperture width 88 of the non-shifter region 832.